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ScienceShot: Ancient Martian Flood

Planetary geologists have long sought to explain regions on Mars where broad areas are littered with shattered, tilted, kilometers-thick chunks of rock. In many cases, these so-called chaotic terrains also seem to be sources of fantastic floods (such as the Aram Chaos crater, upper left; relative elevations range from red [high] to low [blue]). Previously, some teams have proposed that these floods resulted when immense subterranean reservoirs of ice were suddenly melted by the rise of molten material from deep within the Red Planet, and others have suggested that the water was carried from distant regions via aquifers and then somehow abruptly released. A fresh hypothesis suggests that the chaotic terrain in and around Aram Chaos resulted when a buried ice lake trapped in the ancient impact crater suddenly collapsed. In this scenario, the 4.2-kilometer-deep crater started collecting water soon after it formed about 3.5 billion years ago. Then, after the 1.5-km-or-so-deep lake froze, windblown sediments began to accumulate upon its surface. Once that material reached a thickness of 2 kilometers, it blocked the transfer of heat from Mars’s interior, allowing it to melt a large fraction of the buried lake’s ice. Eventually, sometime about 2.5 billion years ago, the weight of the sediments caused the lake’s icy ceiling to collapse, sending immense amounts of water to the surface, the researchers propose online this month in Icarus. Then, that water broke through the crater rim (artist’s concept, lower right), possibly through a weak spot previously eroded by ground water seeping through the rocks at high pressure. Over the course of 30 days or so, possibly less, about 93,000 cubic kilometers of water—almost 80% of the volume of all free-flowing fresh water on Earth today—carved the 15-km-wide, 2.5-km-deep Aram Valley.